Micturition

Question 1

What is the principal nerve supply to the bladder?

Answer 1

From the pelvic nerves (parasympathetic), which connect with the S.C. through the sacral plexus, mainly connecting with cord segments S2-3.

Question 2

What type of nerves are the motor fibres transmitted in the pelvic nerves?

Answer 2

Parasympathetic

Question 3

Describe the sympathetic innervation of the bladder.

Answer 3

• Hypogastric nerve (T12-L2).
• Causes relaxation of the detrusor muscle, promoting urine retention.

Question 4

What is the innervation of the external urehral sphincter?

Answer 4

Skeletal motor fibres transmitted through the pudendal nerve

Question 5

Describe the parasympathetic innervation of the bladder.

Answer 5

• Pelvic nerve (S2-S4).
• Increased signals from this nerve causes contraction of the detrusor muscle, stimulating micturition.

Question 6

Describe how urine is forced to travel from the renal pelvis down to the bladder.

Answer 6

Urine flowing from collecting ducts into renal calyces stretches the calyces and increases their inherent pacemaker activity, which initiates peristaltic contractions that spread to the renal pelvis and then downward along the length of the ureter, thereby forcing urine from the renal pelvis to the bladder.

Question 7

Describe the innervation of the ureters.

Answer 7

Walls of the ureters contain smooth muscle and are innervated by both sympathetic and parasympathetic nerves, as well as by an intramural plexus of neurons and nerve fibres that extends along the entire length of the ureters.

Question 8

What effect do sympathetic and parasympathetic innervations have on the ureters respectively?

Answer 8

• As with other visceral smooth muscle:
  
  • Peristaltic contractions in the ureter are enhanced by parasympathetic stimulation.
  • Peristaltic contractions in the ureter are inhibited by sympathetic stimulation.

Question 9

How is backflow of urine from the bladder prevented?

Answer 9

Ureters enter bladder through detrusor (at trigone) – ureters course obliquely for several cm through the bladder wall. Normal tone of detrusor compresses the ureter, preventing backflow of urine from the bladder when pressure builds up during micturition or bladder compression.

Question 10

What is vesicoureteral reflux?

Why does it occur?

Answer 10

• In some, the distance the ureter courses through the bladder wall is less than normal, and thus contraction of the bladder during micturition does not always lead to complete occlusion of the ureter.
• Therefore, some of the urine in the bladder is propelled backwards into the ureter – vesicoureteral reflux.

Question 11

How are sensory signals from te bladder stretch receptors conducted?

Answer 11

Sensory signals from the bladder stretch receptors are conducted to the sacral segments of the cord through the pelvic nerves and then reflexively back again to the bladder through the parasympathetic nerve fibres by way of these same nerves.

Question 12

Describe the cycle of the micturition reflex.

Answer 12

The micturition reflex is a single complete cycle of:

1. Progressive and rapid increase of pressure
2. A period of sustained pressure
3. Return of the pressure to the basal tone of the bladder

Question 13

Describe how the brain facilitates or inhibits the micturition reflex.

Answer 13

• The micturition reflex is an autonomic S.C. reflex, but it can be inhibited or facilitated by centres in the brain. These centres include:
  
  • Strong facilitative and inhibitory centres in the brain stem, located mainly in the pons.
  • Several centres located in the cerebral cortex that are mainly inhibitory but can become excitatory.
• The micturition reflex is the basic cause of micturition, but the higher centres normally exert final control of micturition as follows:
  
  • The higher centres keep the micturition reflex partially inhibited, except when micturition is desired.
  • The higher centres can prevent micturition, even if the reflex occurs, by tonic contraction of the external urethral sphincter until a convenient time presents itself.
  • When it is time to urinate, the cortical centres can facilitate the sacral micturition centres to help initiate a micturition reflex and at the same time inhibit the external urethral sphincter so that urination can occur.

Question 14

How is voluntary urination initiated?

Answer 14

• First, contraction of abdominal muscles, which increases the pressure in the bladder and allows extra urine to enter the bladder neck and posterior urethra under pressure, thus stretching their walls.
• This action stimulates the stretch receptors, which excites the micturition reflex and simultaneously inhibits the external urethral sphincter.
• Ordinarily, all the urine will be emptied, with rarely more than 5-10mL in the bladder.

Question 15

List 3 abnormalities of micturition.

Answer 15

• Atonic bladder and incontinence cause by destruction of sensory nerve fibres.
• Autonomic bladder caused by S.C. damage above the sacral region.
• Uninhibited neurogenic bladder caused by lack of inhibitory signals from the brain.